1. Field of the Invention
This invention relates to a photoelectric switch with a dewing preventative applied to a front light transmitting plate provided on a body.
2. Description of the Prior Art
A photoelectric switch is utilized as a vision sensor in the FA and other fields, for detecting the presence and position of an object. The photoelectric switch is a superior sensor due to its long detection distance and other characteristics as compared with other contactless switch devices (electromagnetic induction type/electrostatic capacity type contactless switch). However, there is an environmental problem which photoelectric switches must overcome. In particular, it is necessary to solve the problem of erroneous operation due to degraded light transmission from water splashed on the equipment or dew or frost forming thereon.
One type of photoelectric switch solves this problem by providing a photoelectric switch with a heating hood 12 for preventing the formation of dew on a body 11 as shown in FIG. 5 and FIG. 6. FIG. 5 exemplifies a reflection type photoelectric switch, wherein a light emitting part and a light receiving part are incorporated within the body 11. That is, a light such as an infrared ray is projected toward a reflecting plate 13 from the light emitting part in the body 11. A reflected light from the reflecting plate 13 is received in the light receiving part in the body 11. When the projected or reflected light is intercepted by an object 14 to be detected, the interception changes the quantity of received light at the light receiving part. Thus the presence of the object 14 at a predetermined position is detected.
The heating hood 12 has a contour part 15 fitting an outer periphery of the front of the body 11. A light transmitting plate 16 within the contour part 15 covers the front of the body 11. A heating unit 17 (FIG. 6), using carbon resistance, heating wire or the like, is disposed about a peripheral edge portion of the back of the light transmitting plate 16. The heating unit 17 is heated by connection of an electric current thereto from a power source 18 through wires 19. The heat prevents dew from forming on the light transmitting plate 16.
In the above construction, the heating hood 12 is mounted on the front of the body 11 when the photoelectric switch is used in an environment where dew, frost or splashed water or the like light transmission into and out of the body 11. The electric current is connected to the heating unit 17 independently of operation of the body 11, or light projection and reception, and thus asynchronously therewith. For example, connection of electricity to the heating unit 17 may be manually controlled by a user on decision. Alternatively heating may be carried out at all times. A thermostat (not shown) may be mounted within the heating hood 12 for starting and stopping operation of the heating unit whenever temperature passes a preset value.
The light transmitting plate 16 is heated by conduction from the its rear surface on which the heating unit 17 is mounted. Thus, formation of dew or frost on the front of the light transmitting plate 16 is prevented. Splashed water is evaporated from the light transmitting plate 16. by the heat.
As shown in FIG. 6, the heating unit 17 abuts a peripheral edge portion of the back of the light transmitting plate 16. Therefore heat transfer efficiency to the light transmitting plate 16 is poor, especially near its center. Thus the time for removing dew, frost or water from the transmitting portion is unavoidably long. Although the entire light transmitting plate 16 is heated the only portion actually requiring heating is that portion through which the light passes from and to the photoelectric switch. Thus the entire light transmitting plate 16 need not be heated. Accordingly, the efficiency of using heat is low.
The heating hood 12 is a so-called externally fitting type for mounting on the front of the body 11 as occasion demands. The heating unit 17 is operated asynchronously with the body 11 of the photoelectric switch. It is therefore hard to coordinate heating with changes in the environment. That is, as mentioned hereinbefore, energization of the heating unit 17 is controlled manually at the user's decision, or carried out at all times, or is controlled according temperature information on a thermostat. In any case, the problem remains of efficiently responding to changes in the environment wherein a quantity of received light decreases due to dews frost or water and is restored to its original value on removal of the dew, frost or water.